1. Field of the Invention
The present invention relates to radiosurgery and, more specifically, to devices for collimating a beam used in radiosurgery.
2. Description of the Prior Art
As shown schematically in FIGS. 1A-1C, prior art radiosurgery systems that are used to irradiate a mass 4 (such as a tumor, or other diseased tissue) in a body 2 (such as a patient's body) with a high-energy beam 6 employ a gantry-mounted linear accelerator 8 capable of traveling along an arcuate path 10. The linear accelerator 8 directs the beam 6 toward the mass 4 from several different angles and, thus, the mass 4 presents several aspects in relation to the beam 6. For example, when the linear accelerator 8 is in relation to the mass 4 as shown in FIG. 1A, the mass 4 presents its broadest aspect to the beam 6. When the linear accelerator 8 travels along the arcuate path 10, as shown in FIG. 1B, the mass 4 presents a narrower aspect in relation to the beam 6. At some point of travel by the linear accelerator 8 along the path 10, the mass 4 presents its narrowest aspect in relation to the beam 6, as shown in FIG. 1C.
With conventional radiosurgery systems, the beam 6 is collimated so that its is wide enough to irradiate the entire mass 4 and the beam 6 maintains a constant width, regardless of the position of the linear accelerator 8 along the path 10. Thus, when the linear accelerator 8 is in the relation to the mass 4 shown in FIG. 1A, only a narrow portion of the beam 6 unnecessarily irradiates healthy tissue 12' within the body 2. However, as the linear accelerator 8 is in the relation to the mass 4 shown in FIG. 1B, a wider portion of the beam 6 unnecessarily irradiates healthy tissue 12". Once the linear accelerator 8 is in a position along the path 10 where the mass 4 presents its narrowest aspect, as shown in FIG. 1C, a considerable portion of the beam 6 unnecessarily irradiates a substantial amount of healthy tissue 12'".
Unnecessary irradiation of healthy tissue can cause serious harm to the healthy tissue. Therefore, prior art systems have the disadvantage of unnecessarily irradiating substantial amounts of healthy tissue when the relationship of the linear accelerator to the body is such that the mass being irradiated presents a narrow aspect to the beam.
Prior art systems may be stopped periodically to change the collimation of the beam to reflect the current aspect of the mass. However, doing so would introduce the disadvantage of interrupting the treatment process.